I think the fact that the "a" is sandwiched between the specific consonant sounds of the final "r" and beginning "m" causes it to dissolve. If one or both of the consonants were different the "a" wouldn't be so apt to disappear. For instance, "One small step at a man." Following the "t" of "at" the "a" pretty much has to be there, no matter how you try to make it go away. Try it and see.